This invention relates to methods and apparatus for crystallizing chemical species out of a fluid medium such as a solution, a mixture of liquids, or a suspension by indirect cooling of the fluid medium. The crystals can then be separated from the residual fluid medium.
Crystallizable species can be removed from solutions, mixtures of liquids, or suspensions by cooling the fluid medium until crystals form and then separating the crystals from the residual fluid medium. The crystallizable species may be a solvent or a dissolved substance in a solution, or it may be one of the liquids in a mixture or suspension. In some cases, the fluid medium being processed may not be characterized as simply a solution, mixture, or suspension, but may have attributes of two or more of these categories (e.g., a solution or mixture with suspended solids). A precise characterization of the medium being processed is not essential to the invention, and the medium being processed will be referred to in general herein as the fluid whether it is a solution, mixture, suspension, or some combination of these. The residual fluid left by partial crystallization of a fluid will be referred to in general herein as the brine.
Chemical species are crystallized and separated from a fluid either to recover the crystallizable species, or to obtain a concentrated brine, or for both of these purposes. For example, pure water is produced from salt water by removing the solvent from the saline solution. Spent chemical process liquors are concentrated to facilitate reuse or disposal by removing all or a portion of the liquids from the spent liquor. For example, the spent liquor of the wood pulping process in papermaking and similar operations (e.g., the so-called black liquor) is concentrated in this way to allow the brine to burn as a source of energy and to permit recovery of the chemicals therein contained. Various food solutions, suspensions, etc., such as fruit juices, may be concentrated in this way to facilitate handling and further processing.
Austrian Pat. No. 321,244 describes methods and apparatus for removing crystallizable solvents from solutions by indirect cooling of the solution to form crystals of the solvent, followed by mechanical separation of the crystals from the brine. As described in that patent, heat exchange surfaces between the coolant and the solution are vibrated with a component of motion in the plane of those surfaces so that crystals which form on those surfaces are continually broken off and flushed with the brine. This prevents the formation of a continuous layer of crystals adhering to the heat exchange surfaces. Such a layer would reduce the rate of heat transfer between the coolant and the solution, and would have to be removed, e.g., by means of mechanical scrapers. Elimination of srapers greatly simplifies the freezing apparatus and makes it possible to construct such apparatus with much greater heat transfer area per unit volume, e.g., by using many small, closely spaced, parallel heat exchange tubes. Vibration of the heat exchange surfaces also increases the rate of heat transfer between the coolant and the solution by producing turbulence in the coolant and the solution near the heat exchange surfaces.
A possible disadvantage of vibrating the heat exchange surfaces as described above is that a relatively large amount of energy may be required to vibrate the relatively massive structure of the heat exchanger. Vibration of the heat exchanger also subjects it to mechanical stresses which may require it to be more massive than would otherwise be necessary, and which may result in wear and failure of the structure, e.g., as a result of fatigue.
In view of the foregoing, it is an object of this invention to provide improved methods and apparatus for crystallizing chemical species from solutions, mixtures, and suspensions by indirect cooling of the fluid. The crystals can then be separated from the brine.